Can't select bones in pose mode
I had this exact same problem when trying to bring an armature from 2.8 into 2.79, and I found a solution that worked in that case, so maybe it will work for you as well. Try the following:

Create a new additional armature with a single bone in your project file. Then in Object mode, select your problematic armature (the one you can't select bones from in pose mode), shift-select your new single bone armature, and join the two (ctrl-j). 

When I did this I was able to select all of my original bones in pose mode again.